There has been a method for reducing temporal redundancy, a method for reducing spatial redundancy, and a method for reducing signal redundancy as a method for improving encoding efficiency in the conventional image encoding.
A frame difference method or a motion compensation method is adopted as a method for reducing temporal redundancy. In the frame difference method, a simple subtraction between successive two images is performed, and the obtained difference is encoded. In the motion compensation method, a motion vector is applied to a reference frame, so that an approximate image of a frame to be encoded is generated, and a difference between the approximate image and the frame to be encoded is encoded. Since this method reduces a difference between images and encodes the difference, the motion compensation method is more advantageous than the frame difference method in terms of the encoding efficiency. Various methods have been proposed as a method for estimating a motion vector used in the motion compensation method, and a primary conventional technology relating to an estimation of a motion is described in Non-Patent Literature 1.
On the other hand, as a method for reducing spatial redundancy, there is a method for quantizing an orthogonal transformation coefficient. The orthogonal transformation maps a pixel signal to a frequency region so that the energy is concentrated in the lower band. Indiscriminating of visual characteristics of humans to the high band is used and a high band component is removed by the quantization so that the encoding efficiency can be improved. Further, there is a spatial redundancy reduction method by estimation of an orthogonal transformation coefficient or pixel. A primary conventional technology relating to the orthogonal transformation is described in Non-Patent Literature 2.
As a method for reducing signal redundancy, there is a method for reducing a correlation between signals by a principal component analysis. In Patent Literature 1, an encoding processing is applied to a decorrelated signal to generate an encoding signal. In another example of the same Literature, there is also described a method for executing a decorrelation inverse transformation by specifying a decorrelation transformation matrix in accordance with assistant information indicating a unique number of a decorrelation transformation matrix, where the number is decided by selecting the decorrelation transformation matrix closest to a decorrelation transformation matrix evaluated by using a principal component analysis method at an encoding side from a finite number of decorrelation transformation matrices.